1. Field of the Invention
The invention is related to the field of fluid quantification, and in particular, to fluid quantification including fluid flow quantification.
2. Statement of the Problem
Water flow measurement is typically required for municipalities, including measuring run-off water or water in storm drains or sewers. However, the amount of flow can be varying and at times can be unpredictable. Because normal wastewater flows may be added to by storms or other unexpected or uncontrolled events, sewer pipes or sewer systems typically are constructed to be large enough to handle most excessive flow events. Consequently, wastewater conduits are partially filled and act as open channels, absent an unexpectedly high fluid collection. The flow depth typically is only a fraction of the conduit depth most of the time.
Measuring the flow rate of a fluid in an open conduit poses some challenges. The flow rate may vary significantly according to factors such as the slope of the conduit, the roughness of the conduit, the presence of any obstructions and/or changes in direction of the conduit, the nature of the fluid and the viscosity and presence of solid objects or particulate matter in the fluid, and any other interactions between the fluid and the conduit. In addition, the flow depth will play an important part in the flow rate, wherein changes in flow depth will affect the flow velocity, turbulence in the flow, likelihood of obstructions of the flow, etc. Further, a velocity sensor may have dropouts during low flow conditions. For example, a submerged Doppler velocity probe may have dropouts due to insufficient particulates (targets) or the sensor not being submerged.
A flow meter for an open channel flow typically performs a fluid depth measurement (d) and a fluid velocity measurement (v), where the fluid flow rate f is defined as f=(v)(A), where the cross-sectional area (A) may vary with the flow depth (d). The depth (d) is therefore used to determine the area (A). Depending on the instrument, one or more depth probes and one or more velocity probes may be employed. Redundant sensors can be included in order to increase or maintain accuracy.
Depth measurement in an open channel is relatively easy to accomplish. One approach is a pressure measurement, wherein a fluid pressure can be correlated to a fluid depth. Other depth measurements can also be employed, such as an ultrasonic or electromagnetic measurement that directly determines a fluid depth or that indirectly determines fluid depth by determining the distance to the fluid surface.
One type of fluid flow velocity measurement is a measurement of the velocity of the fluid surface. Advantageously, such measurement can be performed non-intrusively, such as from a probe above the fluid surface. For example, the velocity sensor can be located at the top of the conduit or channel and can direct a measurement beam or beams downward onto the fluid surface.
However, measurement of the fluid surface velocity can have difficulties. Objects on the surface can give false readings and can be moving at different velocities than the fluid. For example, the fluid surface can include ripples or waves, solid objects, solid objects that are trapped or hung up (i.e., tree branches, trash, etc.) and can produce noisy or inaccurate readings.
Another type of fluid flow velocity measurement utilizes a submerged ultrasonic probe to make velocity measurements based on Doppler or correlation techniques. Such probes measure the movement of large ensembles of particles carried within the flowing fluid. The resulting velocity measurements may thus better represent the average velocity of the fluid over the channel cross-section.
The potentially better accuracy of submerged probes is balanced by greater difficulties in their use. Because such probes are submerged in a flow which may include high particulate loads, sewage, and corrosive industrial discharges, they may experience degraded function or outright failure due to silting, fouling, and corrosion. These problems sometimes result in velocity measurements which are noisy, inaccurate, or completely missing.
Consequently, a fluid flow rate produced from the fluid velocity measurement will be affected by any inaccuracies or gaps in the fluid velocity measurement.